US secretary of state John Kerry has warned North Korea it would be making a "huge mistake" if it launched one of its medium-range missiles during the current stand-off between it, the United States and South Korea.

Mr Kerry had earlier arrived in Seoul for talks with South Korean president Park Geun-hye, as tensions on the Korean peninsula brew.

"Kim Jong-un needs to understand, as I think he probably does, what the outcome of a conflict would be," Mr Kerry said.

"Kim Jong-un needs to understand, as I think he probably does, what the outcome of a conflict would be. North Korea will not be accepted as a nuclear power.

US secretary of state John Kerry

"If Kim Jong-un decides to launch a missile, whether it's across the Sea of Japan or any other direction, he will be choosing wilfully to ignore the entire international community.

"It will be a huge mistake for him to do that because it will further isolate his country.

"Our hope is we can get back to talks."

Mr Kerry also denounced North Korea's "unacceptable" rhetoric and said it would never be accepted as a nuclear state, while holding out the possibility of dialogue.

"The United States, South Korea and the entire international community are all united in the fact that North Korea will not be accepted as a nuclear power," Kerry added.

"The rhetoric that we are hearing from North Korea is simply unacceptable by any standards."

Mr Kerry also said it was high time for China - whose trade and aid have propped up North Korea since the end of the Cold War - to intervene with its wayward ally if it truly wants to safeguard regional stability.

"China has an enormous capability to make a difference here," he said.

Mr Kerry's warning was closely followed by Japan vowing to respond to any scenario after a threat by North Korea that Tokyo would be "consumed in nuclear flames".

"All we can say is we will take every possible measure to respond to any scenario," a defence ministry official said.

Nuclear power

North Korea has issued weeks of shrill warnings to the US and South Korea, including of waging thermonuclear war, after the imposition of new UN sanctions in response to its third nuclear arms test in February.

Mr Kerry's visit coincides with preparations for the anniversary on Monday of North Korean state founder Kim Il-sung's birth, a possible pretext for a military show of strength.

Speculation has mounted that Pyongyang may launch a medium-range missile after reports in South Korea and the US that missiles had been moved into suitable locations.

Earlier reports that North Korea has a nuclear weapon it can mount on a missile were circulating.

According to an assessment from the US Defence Intelligence Agency – accidentally made public – the US had "moderate confidence" that North Korea may now have some capability to deliver a nuclear warhead via long-range missile.

The reports have since been dismissed by the US and South Korea. When asked if war was imminent, a US official said: "Not at all."

Washington's greatest concern, the official said, was the possibility of unexpected developments linked to the inexperience of North Korea's 30-year-old leader, Kim Jong-un.

"Kim Jong-un's youth and inexperience make him very vulnerable to miscalculation," the official said.

"Our greatest concern is a miscalculation and where that may lead.

"We have seen no indications of massive troop movements or troops massing on the border or massive exercises or anything like that that would back up any of the rhetoric that is going on."

Sceptical

South Korea's defence ministry maintained it did not believe the North could mount a nuclear warhead on a missile.

"North Korea has conducted three nuclear tests but there is doubt whether they're at the stage that they can reduce the weight and miniaturise a device to mount on a missile," South Korea's defence ministry spokesman Kim Min-Seok said.

Its conclusion about North Korea follows more than a month of rising tension on the Korean peninsula.

China, North Korea's only major diplomatic ally, denied reports that it was staging military drills along the North Korean border.

Asked about the US reports that Pyongyang may have developed a nuclear weapon, Chinese foreign ministry spokesman Hong Lei said: "China upholds the maintenance of peace and stability on the Korean peninsula and pushing for its denuclearisation via talks and consultations.

"No matter what changes there are in the situation, we will uphold this direction."